Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Development of a flat panel display device capable of high quality color/gray scale output is an important technological goal. Liquid crystal displays have a number of unique useful characteristics, including low voltage and low power of operation. In such displays, a thin layer of liquid crystal material is placed between glass plates and the optical properties of small domains in the layer are controlled by the application of electric fields with high spatial resolution. This gives a device called a spatial light modulator (SLM), which is an array of pixels which either block or transmit light. The current generation of LCDs utilize the supertwisted nematic cell for displays with high contrast but limited off-axis viewability. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes a preferred orientation in an applied electric field. However, since the coupling to an applied electric field by this mechanism is rather weak, the electro-optical response time of liquid crystal based displays may be too slow for many potential applications such as in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens. Fast optical response times become increasingly important for applications to larger area display devices. Insufficient nonlinearity of liquid crystal based displays can also impose limitations for many potential applications.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N. A. Clark and S. T. Lagerwalll (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
Ferroelectric liquid crystals (FLCs) are fluids possessing thermodynamically stable polar order. As the liquid crystal cools from a normal isotropic liquid (I) to a crystalline state (X), it passes through a series of mesogenic phases of increasing molecular order. Some of these phases include the smectic A (A or SA), and smectic C (C, SC, or SC), or chiral smectic C (C*). Only the smectic C phase possesses the thermodynamically stable polar order necessary to exhibit a net dipole moment. In the smectic C phase the molecules self-assemble into layers, with the long axis of the molecules coherently tilted with respect to the layer normal. The single polar axis of the phase is normal to the tilt plane. For most such FLCs, a spontaneous macroscopic dipole density or spontaneous ferroelectric polarization (P) along the polar axis is easily measurable.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess this spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Meyer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the chiral molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. Fast switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity. The necessary switching speed to achieve a fall color display with temporal gray-scale is about 6xcexc sec.
Birefringence is given by the following equation:
xcex94n=nexe2x88x92no
where ne is the index of refraction along the extraordinary axis of a birefringent material (parallel to the optical axis) and no is the index of refraction along the ordinary axis (perpendicular to the optical axis). Many compounds of the present invention have improved solubility in FLC mixtures containing such compounds, and improved melting temperatures of FLC mixtures containing such compounds. Many compounds of the present invention confer to FLC mixtures containing them decreased viscosity and improved tilt angle.
Another important material characteristic is the birefringence. The birefringence of a compound or composition is the difference in refractive indices between different orientations of the LC.
Birefringence refers to the property of a liquid crystal to interact more strongly with light along one LC axis than along another LC axis. Most LCs are made of a core with extensive electron delocalization, to which one or two tails may be attached to help orient the molecules, give a dipole moment or polarization, or confer other desirable properties on the molecule. Typical LCs are rod-shaped with the majority of the xcfx80 electron delocalization along the long or extraordinary axis (also referred to as the director). As a consequence, the extraordinary axis of LCs have a higher index of refraction than the ordinary axis, so the birefringence (An) is positive. Birefringence of a liquid crystal at a given wavelength is given by:       Δ    ⁢          xe2x80x83        ⁢    n    =            G      ⁢              (        T        )              ⁢                            λ          2                ⁢                  λ                      *            2                                                λ          2                -                  λ                      *                          2              xe2x80x2                                          
where xcex94n is the birefringence at a given wavelength, G is a constant, T is the temperature, xcex is the wavelength, xcex* is the mean resonance frequency which can be calculated using the spectrum of a material, or its birefringence at several wavelengths. See, e.g., S.-T. W (1986) Phys. Rev. A 33:1270; S.-T. W (1987) Opt. Eng. 26:120; S.-T. W, C.-S. W (1989) J. Appl. Phys. 66:5297; S.-T. W et al. (1993) Opt. Eng. 32:1775. As the wavelength of interest moves away from xcex*, the birefringence decreases asymptotically until in the infrared, the birefringence is relatively constant (except in the near-IR portion of the spectrum).
The optimum thickness of the FLC film when used as a half-wave plate (the half-wave thickness d1/2) in the device depends on the birefringence (xcex94n) of the material and light wavelength (xcex) according to the following equation:       d          1      /      2        =      λ          2      ⁢      Δ      ⁢              xe2x80x83            ⁢      n      
Optimal thickness of a FLC film is achieved when the contrast is maximized and true color transmission is exhibited.
Some of the materials with the lowest birefringence currently available possess birefringence around 0.20. This corresponds to a thickness of the FLC for visible light modulation of about 2 xcexcm gm. The use of thinner devices is limited by manufacturing techniques and material characteristics. Manufacturing techniques for large area, very thin devices are expensive and difficult to implement. When thin LC cells are used, small variances in cell thickness can have a significant effect on the cell""s optical properties. For example, a 0.1 xcexcm variance in thickness of a cell that is 1.1 xcexcm thick results in a xc2x19% difference in transmission, whereas the same variance in a thicker 1.9 xcexcm cell results in only a xc2x15% difference in transmission. Thinner LC cells also tend to suffer from non-uniform spacing, which can lead to short circuits. Environmental contamination of LC cells, for example inclusion of dust and other contaminants, has a more severe effect on thinner cells. For flat panel video screens, for example, lower birefringence FLCs would allow thicker cells with lower fabrication costs. In addition, compounds with lower birefringence than currently available materials possess would permit LC cell designs using thicker cells for more stability, easier manufacturing and lower cost. There is a need in the art for LC materials, particularly FLC materials, with decreased birefringence.
Another area where low birefringence FLCs would provide a significant advantage are UV modulators for laser marking applications. At the short wavelengths used in laser marking applications (308 and 349 nm), the device FLC film thickness approaches 1 xcexcm and manufacturability becomes an issue. Decreasing the birefringence by approximately 40% brings the cell thickness back into the range of normal fabrication capabilities.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable liquid crystal materials which exhibit ferroelectric phases (chiral smectic C*) over a substantial temperature range about room temperature. Useful device operating temperatures range from about 10xc2x0 C. to about 80xc2x0 C. More typical device operating temperatures range from 10xc2x0 C. to 30xc2x0 C. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C*) liquid crystal phase. More typically, the device properties of interest can not be achieved in a single compound, but by mixing components of a mixture, the desired device properties can be achieved. Ferroelectric liquid crystal mixtures possessing chiral smectic C* phases with useful temperature ranges can also be obtained by mising chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants, into a liquid crystal host material (which may or may not be composed of chiral molecules). Commercial LC mixtures are generally composed of at least eight components. FLC mixtures generally contain two types of components: 1) a smectic C host, designed to give the mixture the required temperature range and other standard LC properties; and 2) dopant molecules that affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture. These dopants can be chiral components which are designed to induce ferroelectric polarization and produce fast switching or other desirable properties (e.g., tilt angle adjustments) in the FLC film. The dopants can also be achiral components that adjust other desirable FLC properties. The components of FLC mixtures can also be adjusted to vary phase transition temperatures or to introduce desired LC phases.
For modulation of intense light sources, the material must possess low absorption at the wavelengths of interest and high stability to the light transmitted. SLMs designed to modulate TV light, for example, must use materials stable and transparent to UV light.
In addition to the above-described characteristics, the composition of ferroelectric liquid crystal materials can be adjusted to vary the tilt angle, pitch, stability and mixing properties of the FLC materials. Addition of molecules which optimally impart a 22.5xc2x0 tilt angle to an FLC material used in a shutter or light switch, results in maximum throughout in the xe2x80x9cONxe2x80x9d state of the device. A 22.5xc2x0 tilt angle is particularly desirable for FLC materials used in direct drive, flat panel display applications. A longer helix pitch in the smectic C* phase, particularly a pitch longer than about 3.0 xcexcm, is also a desirable characteristic of FLC materials for certain applications, since such a longer helix pitch improves the alignment of the FLC compounds in electro-optical devices, decreases surface interactions and as a consequence improves the usefulness of these compounds in SSFLC (Surface Stabilized Ferroelectric Liquid Crystal) devices. In an SSFLC cell, the orientation of the molecules in the FLC phase is strongly coupled to externally applied fields, due to the presence of a ferroelectric polarization P, and a fast light valve with high contrast, bistability, a sharp threshold, low power requirements and high spatial resolution are achieved. FLC components can also be added which increase the stability of the smectic phases of the FLC material, for example, by suppressing crystallization of FLC materials, and/or improving the miscibility and/or viscosity of the liquid crystal composition. Fabrication of large-area multiplexed matrix addressed arrays of pixels is quite feasible using the properties of the SSFLC geometry.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively xe2x80x9cfloppyxe2x80x9d tails (see Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffmdustrie, Lebzig for a compilation of the molecular structures of LC molecules). FLC materials have been prepared by the introduction of a stereocenter into one of the tails, thus introducing chirality. The first FLC compound to be characterized was DOBAMBC (Meyer et al., supra) which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of xe2x88x923 nC/cm2.
There are a number of reports of compounds containing two or more aromatic rings such as those having phenylbenzoate, biphenyl, phenylpyrimidine, phenylpyridine and related cores coupled to chiral tails which possess smectic C* phases displaying fast switching speeds at room temperature, or which can be employed as FLC dopants to induce high polarization and fast switching speeds when combined in mixtures with FLC host materials. There are also several reports of FLC compounds having cores which contain cyclohexane and cyclohexene rings.
The following are exemplary reports of FLC compounds containing cyclohexane or cyclohexane rings.
Li et al. (1991) Mol. Cryst. Liq. Cryst. 199:379-386 disclose cyclohexenyl liquid crystal compounds, having a chiral center in the mesogenic core, derived from the Diels-Alder reaction between myrcene and methyl acrylate, followed by hydrolysis and esterification with 4-hydroxy-4xe2x80x2-n-alkoxybiphenyl. The liquid crystal materials reported have the following structure: 
The lower member cyclohexenecarboxylates (n=1 and 2) have a large nematic range, and the higher members (n=3-10) have multiple smectic phases in addition to the nematic phase. The presence of the cyclohexene ring is suggested to lead to multiple smectic phases. Nonracemic 4xe2x80x2-n-octyloxybiphenyl 4-(4-methyl-3-pentenyl)-3-cyclohexenecarboxylate is reported to have a smectic C phase and a normal tilt angle, however, its polarization density is extremely small (extrapolated polarization density, Pext, less than 1 nC/cm2). It is suggested that the small polarization density is due to the small dipole associated with the chiral carbon in the tilt plane, which does not contribute to P. Further, the carbonyl adjacent to the chiral produces nearly equivalent, but opposite, dipole moments in the two potential configurations, which occur with nearly equal probability.
Fung et al. (1989) Mol. Cryst. Liq. Cryst. Let. 6(6):191-196 report liquid crystal compounds containing a cyclohexene ring, derived from the Diels-Alder reaction between mycrene and methyl acrylate, followed by hydrolysis and esterification of the resulting acid with 4-hydroxy-4xe2x80x2-methoxybiphenyl or 4-hydroxy-4xe2x80x2-cyanobiphenyl. The liquid crystal materials reported have the following structure: 
where X is a methoxy or cyano group. The two compounds exhibit broad nematic ranges (79-153xc2x0 C. and 93-152xc2x0 C., respectively).
Bezborodov et al. (1989) Liq. Cryst. 4(2):209-215 disclose the mesomorphic (nematic) properties of cyclohexenyl liquid crystal compounds derived from 4-substituted phenols and 4-n-alkylcyclohexene-1-carbonyl chlorides where the double bond is in the 1, 2 or 3 position in the cyclohexene ring. The reference indicates that compounds containing the double bond in the 2 position of the cyclohexene (numbering from the carboxy group as is conventional) are the most promising for use as liquid crystal components, since the appearance of the double bond in the first or 3 positions of the ring causes a large distortion in the shape of the molecule. This distortion reportedly affects both the mesophase (nematic) range and the melting point.
German patent document, Reiffenrath et al., DE 3906040, published Sep. 21, 1989 and WPI Abstract 89-279241/39, refers to cyclohexene derivatives having the general formula:
R1xe2x80x94A1xe2x80x94Z1xe2x80x94A2xe2x80x94(Z2xe2x80x94A3)mxe2x80x94R2
where R1 and R2 are 1-15 carbon alkyl or 3-15 carbon alkenyl groups, optionally with one CN or at least one fluorine or chlorine substituent, in which a CH2 group can be replaced with xe2x80x94Oxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94COOxe2x80x94 or xe2x80x94OCOOxe2x80x94, and one of R1 and R2 can be CN; where A1, A2 and A3 can be 1,4-cyclohexenylene or trans-1,4-cyclo-hexylene in which one or two non-adjacent CH2 groups can be replaced by xe2x80x94Oxe2x80x94, or 1,4-phenylene, optionally with one or two fluorine substituents, in which one or two CH2 groups can be replaced by nitrogen, at least one of A1-3 being 2,3-difluoro-1,4-phenylene, and at least one of A1-3 being 1,4-cyclohexenylene; and where Z1 and Z2 can be xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94 or a single bond. The reference refers to 1,4-cyclohexenylenes having the double bond in the 1, 2 or 3 position.
Tanaka et al. (1989) European Patent Application, Pub. No. 331091 refers to tetracyclic cyclohexylcyclohexene derivatives having the formula: 
where R is a straight-chained alkyl group having 1-9 carbon atoms; A is a cyclohexyl, cyclohexenyl, or phenyl ring; B and C are cyclohexyl or clyclohexenyl rings; n is 0 or 1; when n is 0, X is a cyano group and Y is a hydrogen or fluorine atom; when n is 1, X is a fluorine atom, a straight-chained alkyl group having 1-9 carbon atoms, and Y is a hydrogen or fluorine atom. The disclosed liquid crystal compounds exhibit high Nxe2x80x94I and low Cxe2x80x94N or Sxe2x80x94N points.
Eidenschink et al, WO 87/05015, discloses cyclohexane containing liquid crystal and ferroelectric liquid crystal compositions having the general formulas:
R1xe2x80x94A1xe2x80x94Z1xe2x80x94A2xe2x80x94R2
where A1 and A2 can be a phenyl, cyclohexyl, phenylpyrimidine, or substituted cyclohexene ring. Eidenschink et al. does not specifically disclose a cyclohexene ring, but generically discusses reduced groups at page 14, fourth paragraph, and suggests that the claimed compounds can include reduced groups.
U.S. Pat. No. 5,271,864 (Wand et al.) describes ferroelectric liquid crystal compounds and compositions containing cyclohexyl derivatives of the formula: 
R1 and R2 can be an alkyl, cycloalkyl, alkenyl, alkoxy, thioalkyl, or alkylsilyl group having from one to about 20 carbons. Y is an ester or ether group. Ar1 and Ar2, independent of each other, can be phenyl rings, halogenated phenyl rings, and nitrogen-containing aromatic rings. Ar1 and Ar2 are not separated by a linker.
While a number of useful liquid crystal and smectic liquid crystal materials (both pure compounds and mixtures) have been reported, there is a growing need for LC and FLC materials with varying properties of pitch and tilt angle for use in varied applications. In order to obtain faster switching speeds, FLC materials with low orientational viscosity are desirable. Further, there is a need for LC host materials and FLC dopants with varying mixing properties (which are dependent, at least in part, on chemical composition) for use in the preparation of FLC mixtures having desired chiral smectic phases at useful device operating temperatures (e.g. about 0xc2x0-100xc2x0 C., preferably around room temperature about 10xc2x0-35xc2x0 C.). LC and FLC materials which result in mixtures that are stable to crystallization over useful device operating temperatures are desirable. LC host materials and FLC dopants which are readily synthesized and which possess low birefringence, impart longer chiral smectic phase pitch, tilt angle of about 22.5xc2x0, lower orientational viscosity, broader LC and FLC phases, and suppress crystallization in such mixtures are of particular interest.
The present invention provides new LC compounds that have low birefringence (i.e., birefringence of 0.17 or less, preferably 0.15 or less) and liquid crystal mixtures containing such compounds. The compounds have core groups containing cyclohexyl and/or cyclohexenyl rings. Compounds of this invention have improved solubility in mixtures containing them. Mixtures containing these compounds have improved properties, including broader temperature range for the C* phase (lower low end temperature and/or higher high end temperature), improved (lower) viscosity, and in some cases higher tilt angle. Some of the compounds of the present invention have a C* phase and some have a B* but no C* phase. Some of the compounds of the present invention contain a stereocenter and are chiral, others do not contain a stereocenter and are not chiral. Mixtures which do not contain a chiral or nonracemic component are not ferroelectric. Those compounds which are not chiral are preferably doped with a chiral material to yield a ferroelectric mixture.
The present invention includes compounds of the formula:
R1xe2x80x94Arxe2x80x94R2
wherein
R1 and R2, independently of one another, are selected from the group consisting of alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, alkoxy, thioalkyl, thioether and ether groups having from 1 to about 20 carbon atoms, wherein one or more xe2x80x94CH2xe2x80x94 groups in R1 and R2 may be independently replaced with a member of the group consisting of O, S, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94CH2Sxe2x80x94, xe2x80x94SCH2xe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SCOxe2x80x94, a double bond, a triple bond, and an alkyl silyl group, Si(Re)(Rf), in which Re and Rf, independently of one another, are small alkyl or alkenyl groups having from about 1 to about 8 carbon atoms, said R1 or R2, independently of one another, can be partially or fully halogenated, and said R1 or R2, independently of one another, can be internally or terminally branched;
Ar is a ring core moiety of the general formula: 
xe2x80x83wherein Cyc is a cyclohexyl ring or a cyclohexenyl ring, which may be optionally substituted with members selected from the group consisting of halogen atoms, methyl groups, trifluoromethyl groups, cyano groups, methoxy groups, and trifluoromethoxy groups; RX, RY and RZ, independently of one another, are aromatic or non-aromatic rings which are selected from the group consisting of 1,4-phenyl, 2,5-pyridinyl, 2,5-pyrimidinyl, 2,5-pyrazinyl, 3,6-pyridazinyl, 2,5-dithiazolyl, 1,4-cyclohexyl, 1-4-cyclohex-2-enyl, and 1,4-cyclohexenyl rings, wherein any hydrogen in RX, RY and RZ may optionally be substituted with a halogen atom; wherein x, y, z are, independently of one another, 0, 1, or 2 such that x+y+z=1, 2, or 3; wherein A, B and D, independently of one another, are linkers selected from the group xe2x80x94(CH2)wxe2x80x94, wherein w is 0 to about 8, and wherein one or more xe2x80x94CH2xe2x80x94 groups in A, B or D may be independently replaced with a member selected from the group consisting of O, S, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OOCxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94SCH2xe2x80x94, xe2x80x94COSxe2x80x94, xe2x80x94SCO xe2x80x94, a double bond, a triple bond, and an alkyl silyl group, Si(Re)(Rf), in which Re and Rf, independently of one another, are small alkyl or alkenyl groups having from about 1 to about 8 carbon atoms; wherein a, b, d, independently of one another, are 0 or 1, such that a+b+d=1, 2, or 3.
In general, suitable liquid crystal cores (Ar) are rigid, linear moieties. Preferred cores are those that are chemically stable and which do not impart high orientational viscosity in the liquid crystal phase. Cores of the present invention contain either a cyclohexyl or cyclohexenyl ring which is also bonded, preferably at the para position from its bond to other core moieties, to a branched or unbranched R1.
Exemplary RX, RY and/or RZ rings of the Ar core include, but are not limited to, the following: 
Any of RX, RY and/or RZ rings of the Ar core may optionally be partially or fully halogenated. If halogenated, preferably fluorine is the halogen. Preferred halogenated RX, RY and RZ rings are fluorinated 1,4-phenyl rings with 2-fluoro, 3-fluor or 2,3-difluoro substituted 1,4-phenyl rings and 2-fluoro, 3,6-substituted pyridine rings being more preferred.
Preferred Ar core moieties of this invention include 1,4-substituted phenyl rings, halogenated 1,4-substituted phenyl rings, phenylpyridinyl rings, pyridinylphenyl rings, phenylpyrimidinyl rings, and pyrimidinylphenyl rings.
Tails R1 and R2 are preferably linked on opposite ends of the core in a para arrangement. The rings of the core other than the Cyc ring can be arranged within the core in either orientation with respect to the Cyc ring and the R1 tail.
The compounds of the present invention have R2 which may or may not be chiral. R2 tails of the present invention include alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, alkoxy, ether, thioalkyl, thioether, or alkylsilyl groups having one to twenty carbon atoms. The R2 tails can be straight-chain or branched. Alkenyl R2 tails preferably have one double bond. R2 tails include alkoxy tails, e.g., R2=CnH2n+1xe2x80x94Oxe2x80x94 (where n is preferably 20 or less) and ether tails, e.g., CnH2n+1xe2x80x94Oxe2x80x94CmH2m+1xe2x80x94 (where nand m are preferably 19 or less) and preferably contain one oxygen atom. R2 tails include thioalkyl tails, e.g., R2=CnH2n+1xe2x80x94Sxe2x80x94 (where n and m are preferably 20 or less), and thioether tails, e.g., R2=CnH2n+1xe2x80x94Sxe2x80x94CmH2m+1xe2x80x94 (where n and m are preferably less than 19), and preferably contain one sulfur atom. R2 tails also include allylsilyl tails, e.g., CnH2n+1xe2x80x94Si(CH3)2xe2x80x94CmH2m+1xe2x80x94 (where n and m are preferably 18 or less) or (CH3)3Sixe2x80x94CnH2n +1xe2x80x94 (where n is preferably 17 or less), where a dialkylsilyl group such as (CH3)2Si is inserted within an alkyl chain. R2 cycloalkyl tails include cyclopropyl tails, particularly wherein a cyclopropyl group is at the end of the tail (xcfx89-position), e.g., c-propyl-CnH2n+1xe2x80x94 (where n is preferably 17 or less). Preferred R2 tails have one to twenty carbon atoms, i.e., nxe2x89xa620 in the above exemplified formulas. R2 tails of the present invention are most preferably alkyl, alkoxy and xcfx89-alkenyl tails containing one to twenty carbon atoms. Non-adjacent carbon atoms in R2 alkyl, alkoxy or alkenyl tails can be replaced with a double bond, oxygen atom, sulfur atom, cyclopropyl group or silylalkyl group such as Si(CH3)2. Preferred tails contain only one such substitution. R2 tails more preferably contain three to twelve carbon atoms and most preferably five to twelve carbon atoms. Preferred tails contain trans alkenyl groups.
In general, R1 can be any alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, alkoxy, ether, thioalkyl, thioether, or alkylsilyl groups as they are defined above for R2. R1 and R2 can be the same or different tail groups. As with R2, R1 tails can be straight-chain or branched, chiral nonracemic or achiral groups. Preferred R1 tails contain 1 to 20 carbon atoms. Tails having three to twelve carbons are more preferred and tails having five to twelve carbons are most preferred. In any compounds of this invention wherein Cyc is 1,4-cyclohexenyl, the double bond of the cyclohexenyl ring can be at any position in the ring: 1, 2, 3, 4, 5, or 6. These various configurations are shown below. In the diagrams below, xe2x80x94Yxe2x80x94) indicates the remainder of the molecule. 
Of the compounds of this invention which contain a cyclohexenyl ring, preferred compounds are those with the double bond in the cyclohexenyl ring at the position attached to the tail at position 4. In compounds wherein R1 is attached to the cyclohexenyl ring at the double bond, preferred R1 tails have a CH2 group at the first position in the tail.
R1 that are alkyl and alkenyl groups are more preferred. For R1 that is a thioether or ether, tails containing a single S or O are preferred, such as CnH2n+1xe2x80x94Sxe2x80x94CmH2m+1 and CnH2n+1xe2x80x94Oxe2x80x94CmH2m+1 (where n and m are preferably 19 or less). In alkyl, alkenyl, thioether and ether R1 tails, one or more of the non-neighboring carbon atoms can be replaced with a cyclopropyl group, alkylsilyl group, S atom or O atom. Preferably, R1 groups contain only one such substitution and such substitution is preferably not at the 1-position in the tail.
If R2 or R1 is an alkenyl group, the double bonds can be located at any position in the group and can be cis or trans substituted double bonds. However, trans double bonds are preferred over cis double bonds which are likely to result in reduced solubility of the compound in host materials. Additionally, cis double bonds in R1 and R2 tails will likely narrow the smectic C* range.
R1 and R2 can be straight chain or branched. Branching of R1 and/or R2 can broaden the smectic C* phase of the compound itself or of an FLC mixture containing the compound. The branching effect is enhanced when branching is more distant from the core. It has also been observed that if branching occurs at carbons 2-8 (relative to the core), the polarization density of the FLC molecule is generally not significantly affected.
Specific R1 and/or R2 groups include, but are not limited to: methyl, ethyl, n-propyl, iso-propyl, n-butyl, s-butyl, iso-butyl, t-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1,1-dimethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, n-heptyl, 1-methyhexyl, 2-methylhexyl, 3-methylhexyl, 4-methylhexyl, 5-methylhexyl, n-octyl, 1-methylheptyl, 2-methylheptyl, 3-methylheptyl, 4-methylheptyl, 5-methylheptyl, 6-methylheptyl, n-nonyl, 1-methyloctyl, 2-methyloctyl, 3-methyloctyl, 4-methyloctyl, 5-methyloctyl, 6-methyloctyl, 7-methyloctyl, n-decyl, 1-methylnonyl, 2-methylnonyl, 3-methylnonyl, 4-methylnonyl, 5-methylnonyl, 6-methylnonyl, 7-methylnonyl, 8-methylnonyl, dimethylpentyl, dimethylhexyl, dimethylheptyl, dimethyloctyl, dimethylnonyl, n-undecyl, n-dodecyl, dimethyldecyl, n-propadecyl, n-butadecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, 3-butenyl, 4-pentenyl, 5-hexenyl, 6-heptenyl, n-propoxy, n-ethoxy, n-butoxy, n-undecoxy, n-dodecoxy, 2-methoxymethyl, 2-methoxypentyl, 2-oxypentyl, 3-oxypentyl, 4-oxypentyl, 2-oxyhexyl, 3-oxyhexyl, 4-oxyhexyl, 5-oxyhexyl, 2-oxyheptyl, 3-oxyheptyl, 4-oxyheptyl, 5-oxyheptyl and 6-oxyheptyl, n-5-hexenyl, n-6-heptenyl, n-7-octenyl, n-8-nonenyl, n-9-decenyl, 4-methyl-3-pentenyl, 5-methyl-4-hexenyl, n-8-cyclopropyloctyl, n-7-cyclopropylheptyl, 6-trimethylsilylhexyl, 7-trimethylsilylheptyl, 8-trimethyl-silyloctyl, n-butyldimethylsilylbutyl.
Formulas for R1 and/or R2 groups include, but are not limited to the following (where nxe2x89xa620):
CnH2n+1-(alkyl), nxe2x89xa620
CnH2n+1O-(alkoxy), n xe2x89xa620
CnH2n+1S-(thioalkyl), nxe2x89xa620
xe2x80x83CH2xe2x95x90CHxe2x80x94CnH2n+1-(alkene), nxe2x89xa618
c-propyl-CnH2n+1-(cyclopropylalkyl), nxe2x89xa617
CnH2n+1xe2x80x94Oxe2x80x94CmH2m+1-(ether), nxe2x89xa619, mxe2x89xa619
CnH2n+1xe2x80x94Sxe2x80x94CmH2m+1-(thioether), nxe2x89xa619, mxe2x89xa619
(CH3)3xe2x80x94Sixe2x80x94CnH2n+1-(trimethylsilylalkyl), nxe2x89xa617
The invention provides compounds of Formula IIIA: 
In Formula IIIA, A is as defined above and is preferably an ester, e.g. xe2x80x94COOxe2x80x94. In Formula IIIA, the phenyl ring can be halogenated, preferably with fluorine, and, if halogenated, is preferably mono- or di-halogenated. In Formula IIIA, b is as defined above and is preferably an even integer and more preferably 2 or 4, and such that the linker between the phenyl and the cyclohexyl rings is, for example, preferably (CH2)2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, or xe2x80x94(CH2)2xe2x80x94CHxe2x95x90CHxe2x80x94. R1 and R2 are tails as defined above and are preferably alkyl or alkenyl having from 1 to about 20 carbons atoms, and more preferably are small alkyl or small alkenyl having from 1 to about 8 carbon atoms. R1 and R2 tails can be branched or unbranched. For example, R1 can be 4-methyl-3-pentenyl, and R2 can be xe2x80x94(CH2)4CH3 or xe2x80x94Oxe2x80x94(CH2)4CH3.
The cyclohexyl ring of Formula IIIA can be replaced with a cyclohexenyl ring, to yield compounds of Formula IIIB: 
The double bond of either cyclohexenyl ring can be in any of the 6 possible positions. For example following structures show the double bond of the cyclohexenyl ring attached to R2 in various positions: 
The invention further provides compounds of Formula IVA: 
In Formula IVA, A is as defined above and is preferably an ester, e.g. xe2x80x94COOxe2x80x94. The phenyl ring can be halogenated, preferably with fluorine, and, if halogenated, is/are preferably mono- or di-halogenated. In Formula IVA, b is as defined above and is preferably an even integer and more preferably 2 or 4, and such that the linker between the phenyl and the cyclohexyl rings is, for example, preferably xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, or xe2x80x94(CH2)2xe2x80x94CHxe2x95x90CHxe2x80x94. R1 and R2 are tails as defined above and are preferably alkyl or alkenyl having from 1 to about 20 carbons atoms, and more preferably are small alkyl or small alkenyl having from 1 to about 8 carbon atoms. R1 is branched, preferably terminally branched, e.g. at the carbon farthest from the core. R2 can be branched or unbranched. For example, R1 can be 4-methylpentyl, and R2 can be xe2x80x94(CH2)4CH3 or xe2x80x94Oxe2x80x94(CH2)7CH3.
The cyclohexyl ring attached to R2 can be replaced with a cyclohexenyl ring to yield compounds of Formula IVB: 
The invention further provides compounds of Formula VA: 
In Formula VA, A is as defined above and is preferably an ester, e.g. xe2x80x94COOxe2x80x94. The phenyl and/or pyrindinyl rings can be halogenated, preferably with fluorine, and, if halogenated, is/are preferably mono- or di-halogenated. The order of the phenyl and pyridinyl rings can be reversed. R1 and R2 are tails as defined above and are preferably alkyl, alkenyl or alkoxy having from 1 to about 20 carbons atoms, and more preferably are small alkyl, small alkenyl or small alkoxyl having from 1 to about 8 carbon atoms. R1 is branched, preferably terminally branched, e.g. at the carbon farthest from the core. R2 can be branched or unbranched. For example, R1 can be 4-methylpentyl, and R2 can be xe2x80x94(CH2)7CH3 or xe2x80x94Oxe2x80x94(CH2)7CH3.
The pyridinyl ring of Formula VA can be replaced by a phenyl ring, so that the core contains a biphenyl system, yielding compounds of Formula VB: 
The invention further provides compounds of Formula VIA: 
In Formula VIA, A is as defined above and is preferably an ester, e.g. xe2x80x94COOxe2x80x94. The phenyl and/or pyrimidinyl rings can be halogenated, preferably with fluorine, and, if halogenated, is/are preferably mono- or di-halogenated. The order of the phenyl and pyrimidinyl rings can be reversed, i.e., the phenylpyrimidinyl group can be replaced by a pyrimidinylphenyl group. R1 and R2 are tails as defined above and are preferably alkyl, alkenyl, alkoxyl or ether groups having from 1 to about 20 carbons atoms, and more preferably are small alkyl, small alkenyl, small alkoxyl or small ether groups having from 1 to about 8 carbon atoms or medium alkyl, medium alkenyl, medium alkoxyl or medium ether groups having from about 9 to about 15 carbon atoms. R1 and R2 can be branched or unbranched. For example, R1 can be xe2x80x94C5H11xe2x80x94, CH3xe2x80x94CH2xe2x80x94Oxe2x80x94(CH2)4xe2x80x94 or C4F9xe2x80x94(CH2)4xe2x80x94, and R2 can be C10H21 or xe2x80x94Oxe2x80x94C10H21. As explained above, the double bond of the cyclohexenyl ring can be at any position in the ring.
The pyrimidinyl ring of Formula VIA can be replaced with a phenyl ring such that the core contains a biphenyl system, yielding compounds of Formula VIB: 
The invention also provides compounds of Formula XC: 
In Formula XC, B is as defined above and is preferably an ester, e.g. xe2x80x94COOxe2x80x94. In Formula XC, the phenyl ring can be halogenated, preferably with fluorine, and if halogenated, is preferably mono- or di-halogenated. In Formula XC, b is greater than 0, preferably 1. R1 and R2 are tails as defined above and are preferably alkyl or alkenyl having from 1 to about 20 carbon atoms. R1 and R2 tails can be branched or unbranched.
The linker B of Formula XC can be absent, and the second cyclohexyl ring after R1 of Formula XC can be replaced with an aromatic ring, to yield compounds of Formula XA: 
In Formula XA, the phenyl rings can be halogenated, preferably with fluorine, and if halogenated, is preferably mono- or di-halogenated. In Formula XE, R1 and R2 are tails as defined above and are preferably alkyl or alkenyl having from 1 to about 20 carbon atoms. R1 and R2 tails can be branched or unbranched and can contain oxygen. R1 and R2 can also be halogenated, preferably with fluorine.
The second cyclohexyl ring after R1 of Formula XC can be replaced with an aromatic ring, and a linker added between said aromatic ring and the first cyclohexyl ring after R1 of Formula XC, to yield compounds of Formula XE: 
In Formula XE, A is as defined above and is preferably xe2x80x94(CH2)xe2x80x94. In Formula XE, a is as defined above and is preferably an even integer and more preferably 2 or 4, such that the linker between the cyclohexyl ring and the first aromatic ring after R1 is, for example, preferably xe2x80x94(CH2)2xe2x80x94, or xe2x80x94(CH2)4xe2x80x94. In Formula XE, B is as defined above and preferably contains oxygen, for example, xe2x80x94COOxe2x80x94 or xe2x80x94O(CH2)xe2x80x94. In Formula XE, b is defined above and is greater than 0, preferably an even integer and more preferably 2 or 4. In Formula XE, the phenyl rings can be halogenated, preferably with fluorine, and if halogenated, is preferably mono- or di-halogenated. In Formula XE, R1 and R2 are tails as defined above and are preferably alkyl or alkenyl having from 1 to about 20 carbon atoms. R1 and R2 tails can be branched or unbranched and can contain oxygen.
In Formula XE, only one linker and only one aromatic ring can be present, to yield compounds of Formula XB. 
In Formula XB, A is as defined above and is preferably xe2x80x94(CH2)xe2x80x94. In Formula XE, a is as defined above and is preferably an even integer and more preferably 2 or 4, such that the linker between the cyclohexyl ring and the first aromatic ring after R1 is, for example, preferably xe2x80x94(CH2)2xe2x80x94, or xe2x80x94(CH2)4xe2x80x94. In Formula XE, the phenyl ring can be halogenated, preferably with fluorine, and if halogenated, is preferably mono- or di-halogenated. In Formula XE, R1 and R2 are tails as defined above and are preferably alkyl or alkenyl having from 1 to about20 carbon atoms. R1 and R2 tails can be branched or unbranched and can contain oxygen or halogen atoms.
In the compounds of the invention, the linker between the cyclohexenyl ring and the aromatic ring is preferably described as: 
where X can be H2or O.
Preferred compounds of the invention include compounds of the formula R1 xe2x80x94Arxe2x80x94R2, wherein Ar is a ring core moiety of the general formula: 
wherein at least one of R1 and R2 is terminally or internally branched or contains at least one atom other than carbon or hydrogen, and at least one of A, B and D contains at least one oxygen atoms, and one or more of RX, RY and RZ is aromatic.
Also preferred are compounds of the formula R1xe2x80x94Arxe2x80x94R2, wherein Ar is a ring core moiety of the general formula: 
wherein Cyc is a cyclohexyl ring, RX is an aromatic ring, RY is a cyclohexyl ring, A is xe2x80x94COOxe2x80x94, a, b, x, and y are 1, d and z are 0, R1 is a terminally branched alkyl group, having from about 1 to about 10 carbon atoms optionally partially or fully halogenated (e.g., fluorinated) and optionally containing oxygen, R2 is a straight chain alkyl group, having from 1 to about 12 carbon atoms, B is xe2x80x94(CH2)wxe2x80x94, wherein w is 1 to about 6.
Also preferred are compounds of the formula R1xe2x80x94Arxe2x80x94R2, wherein Ar is a ring core moiety of the general formula: 
wherein Cyc is a 1,4-cyclohexenyl ring, RX is an aromatic ring, RY is a 1,4-cyclohexyl ring, A is xe2x80x94COOxe2x80x94, a, b, x and y are 1, d and z are 0, R1 is a terminally branched alkyl group optionally, partially or fully halogenated (fluorinated), and optionally containing oxygen, having from about 1 to about 10 carbon atoms, R2 is a straight chain alkyl group, having from 1 to about 12 carbon atoms, B is xe2x80x94(CH2)wxe2x80x94, wherein w is 1 to about 6.
Also preferred are compounds of the formula R1xe2x80x94Arxe2x80x94R2, wherein Ar is a ring core moiety of the general formula: 
wherein Cyc is a 1,4-cyclohexenyl ring, RX is a 2,5-pyrimidinyl ring, RY is an aromatic ring, A is xe2x80x94COOxe2x80x94, a, x and y are 1, R1 is a straight chain alkyl group, having from about 1 to about 10 carbon atoms, optionally partially or fully halogenated with fluorine and optionally containing oxygen, R2 is a straight chain alkyl group, having from 1 to about 12 carbon atoms, b, d and z are 0, and wherein the xe2x80x94CH2xe2x80x94 group of the R2 alkyl chain bonded to RY is replaced with xe2x80x94Oxe2x80x94.
Also preferred are compounds of the formula R1xe2x80x94Arxe2x80x94R2, wherein Ar is a ring core moiety of the general formula: 
wherein Cyc is a 1,4-cyclohexenyl ring, RX is an aromatic ring, RY is a 2,5-pyrimidinyl ring, A is xe2x80x94CH2Oxe2x80x94, a, x and y are 1, R1 is a terminally branched alkenyl group, having from about 1 to about 10 carbon atoms, R2 is a straight chain alkyl group, having from 1 to about 12 carbon atoms partially halogenated with fluorine, b, d and z are 0, and wherein the xe2x80x94CH2xe2x80x94 group of the R2 alkyl chain bonded to RY is replaced with xe2x80x94Oxe2x80x94.
Also preferred are compounds of the formula R1xe2x80x94Arxe2x80x94R2, wherein Ar is a ring core moiety of the general formula: 
wherein Cyc is a 1,4-cyclohexyl ring, RX is an aromatic ring, RY is a 2,5-pyrimidinyl ring, A is xe2x80x94COOxe2x80x94, a, x and y are 1, b, d and z are 0, R1 is a terminally branched alkyl group, having from about 1 to about 10 carbon atoms, R2 is a straight chain alkyl group, having from 1 to about 15 carbon atoms.
This invention includes LC and FLC compositions and FLC host compositions having a low birefringence (i.e., 0.17 or less, preferably 0.15 or less) containing one or more of the compounds of Formulas IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, XA, XB, XC and XE.
The compounds of the present invention contain at least one cyclohexyl or cyclohexenyl ring and exhibit low birefringence (0.17 or less, preferably 0.15 or less measured using an Abbe refractometer). Birefringence is given by the following equation:
xe2x80x83xcex94n=nexe2x88x92no
where ne is the index of refraction along the extraordinary axis of a birefringent material (parallel to the optical axis) and n0 is the index of refraction along the ordinary axis (perpendicular to the optical axis). Many compounds of the present invention have improved solubility in FLC mixtures containing such compounds, and improved melting temperatures of FLC mixtures containing such compounds. Many compounds of the present invention confer to FLC mixtures containing them decreased viscosity and improved tilt angle. Tilt angle is measured using optical microscopy. Rise time is used to determine viscosity.
Preferred examples of the compounds and liquid crystal mixtures of this invention include but are not limited to the illustrated examples shown.
Formula IIIA encompasses the following non-limiting examples. 
Non-limiting examples of compounds of Formula IIIB include the following: 
Formula IVA encompasses the following non-limiting examples: 
Non-limiting examples of compounds of Fonnula IVB include the following: 
Formula VA encompasses the following non-limiting examples: 
RX and RY of Formula VA can be optionally substituted with 2 nitrogen to give representative compounds: 
Non-limiting examples of compounds of Formula VB include the following: 
Formula VIA encompasses the following non-limiting examples: 
Non-limiting examples of the compounds of Formula VIB include the following: 
Formula XA encompasses the following non-limiting examples: 
Formula XE encompasses the following non-limiting examples. 
Formula XB encompasses the following non-limiting examples. 
The compounds of the invention encompass examples other than those specifically illustrated. For example, the invention encompasses various ring substitutions and configuration, including compounds as those when Cyc is a 1,4-cyclohexyl ring, then any of RX, RY or RZ can be unhalogenated 2,5-pyridinyl. Also, compounds with one or two 1,4-cyclohexyl rings are included in the invention.
When any of RX, RY or RZ (but not all of RX, RY and RZ) are R1xe2x80x94Arxe2x80x94R2, 2,3-difluorophenyl, then preferred compounds include those in which none of RX, RY or RZ is 1,4-cyclohexenyl or trans-1,4-cyclohexyl.
The linkers are also extremely variable. For example, a variety of lengths of linkers (A, B, D) can be present. In certain cases, when Cyc is a 1,4-cyclohexyl group, and when A, B or D is represented by xe2x80x94(CH2)wxe2x80x94, w can be 0 to 8, preferably not 4, and any xe2x80x94CH2xe2x80x94 group can be substituted with a variety of groups.
The linker A can be further limited in compounds of the invention. For example, when Cyc is a 1,4-cyclohexenyl or 1,4-cyclohexyl ring, and b, d, and z are 0, and when A contains 1 or 2 oxygen atoms, compounds of the invention include those where at least one of R1 and R2 contains one or more halogen atoms, preferably fluorine.
In general, the compounds of this invention have lower viscosity and greater solubility in liquid crystal host mixtures, as well as lower low temperature for the C* phase than those previously reported. These properties are shown by Tables 1-4. Table 1 lists the tilt angle, switching time, crystallization temperature and melting point for some of the compounds of the invention. Table 2 lists the polarization, host material used for the polarization measurement, concentration of the dopant in the host, tilt angle of the mixture, and the rise times of the mixture. The rise time is inversely proportional to the polarization of the mixture. A high polarization will therefore not always result in a short response time, as the compound may have a higher than average viscosity. Smaller tilt angles lead to shorter optical responses, but a tilt angle of less than 22.5xc2x0 leads to lower than optimal contrast ratios.
The phase diagrams for some of the compounds of the invention are shown in Tables 3-4. It is preferred that a compound have a C phase, since the presence of a C phase in the compound widens the C phase of a mixture incorporating the compound. The lack of a C phase does not indicate the compound is not useful in a particular application, since the components and amounts of compounds used in mixtures used can be adjusted to obtain the phases desired. Table 5 illustrates the composition of the hosts used in the measurements. Table 6 shows representative birefringence data.
There are certain structure-property correlations that are seen in the compounds of the invention. Compounds with partially or fully fluorinated tails exhibit phase diagrams similar to those of materials having more rings. For example, when R1 and/or R2 are substituted with F, the viscosity of the compound is lower than if no fluorine is present. In addition, partially fluorinated tails on two- or one-ring compounds provide mesogenic phases similar to that present in three-ring compounds. Partially perfluorinated tails tend to increase the phase transition temperatures, similar to the behavior seen when an additional aromatic ring is found in the compound with less increase in viscosity than a compound with an additional aromatic ring. In compounds that contain three aromatic rings are more likely to have a smectic C phase, but the viscosity and birefringence are higher than compounds with fewer rings exhibit. Compounds with two aromatic rings have lower viscosity and lower birefringence than compounds with three aromatic rings. Compounds with two non-conjugated rings have even lower birefringence than compounds with two conjugated rings. When the compound contains cyclohexyl rings, the ring is more xe2x80x9cfloppyxe2x80x9d and less easily stacked than aromatic rings. However, cyclohexyl rings in a molecule act similarly to an aromatic ring with respect to the properties of the compound. Compounds that contain only two non-conjugated rings generally have even lower birefringence than compounds with two conjugated rings. Solubility improvement depends on the host into which dopant is to be dissolved and on dopant structure. Phenylpyrimidine hosts are preferred over phenylbenzoate hosts.
Compounds with two cylcohexyl rings or one cyclohexyl and one cyclohexenyl ring (for example, Groups IIIA and IVA), give a higher ordered smectic B phase than would otherwise be expected. The smectic B phase in these compounds is similar to a smectic A phase in other molecules.
Cyano substitution on the Cyc ring helps increase the dielectric constant of the material (for example MDW 1091, 1086). A fluoromethyl tail such as seen in compounds 1091 and 1086 has lower twisting power than a difluoro tail, and has lower polarization than a difluoro tail. A compound with a monofluoro tail tends to have polarization similar to that of a fluoromethyl compound.
Another important physical characteristic of a compound is the ultraviolet (UV) absorption and UV stability. In certain applications, it is desired to use compounds that have low UV absorption and high stability. In general, the more aromatic rings the molecule has, the more the compound absorbs UV light. Fewer conjugated rings that are present in the molecule should also lower UV absop tion. Certain functional groups such as a hydroquinone ring or a diacetylene are particularly unstable to UV light.
In addition, compounds with the following structure: 
exhibit lower birefringence and viscosity than compounds with the following structure present in the molecule: 
The tails also contribute to the characteristics of the molecule. Generally, straight chain or branched alkyl chains leads to increased tilt angle as compared to tails that are ethers or contain F.
In general, the presence of oxygen in the tails gives a wider smectic C phase than branched or straight chain alkyl chains in the tails. Also, more aromatic rings present in the molecule generally give a wider phase than fewer aromatic rings present in the molecule. In addition, compounds containing a connector between RX and RY have a narrower smectic C phase than compounds having no connector between RX and RY. When an unsubstituted phenyl ring is present for RX, RY or RZ, the smectic C phase is wider than a phenyl ring substituted with 2 fluorine molecules.
In general, the melting point of the molecule is higher for straight chain alkyl tails with no fluorine substitutions than if fluorine is substituted on the tails.